Pb-Pb

There are 4 stable isotopes of Pb: one non-radiogenic isotope (204Pb) whose abundance hasn’t changed since solar system formation, and three (206Pb, 207Pb and 208Pb) whose abundance has increased since the time of solar system formation by addition of radiogenic contributions from the decay of isotopes of U and Th.

Measurement of the Pb isotopic composition within a rock or mineral— in particular, the abundance of the Pb isotopes effected by radioactive decay relative to that of the non-radiogenic isotope 204Pb (i.e. the ratios 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb), can provide information about the rock or mineral’s age.

Common-Pb method

The Earth is assumed to have an initial Pb composition identical with that of chondritic meteorites. The most primitive Pb isotopic composition measured is that of Canyon Diablo meteorite.

The Geochron is the isochron that includes meteorites and the Earth. The Earth has since differentiated into core, mantle and continental crust, which each have different U/Pb and Pb isotopic compositions. The modern-day basalt field overlaps with Geochron but extends to right, indicating progressive gain of U relative to Pb.

Some minerals, such as galena (PbS), concentrate Pb in their mineral structure and have very low U/Pb. Their Pb isotopic compositions are thus “frozen” at time of mineral formation. The Pb isotopic compositions of these low U/Pb minerals can be used to approximately date the time that the mineral formed. This method is not very accurate because the Pb isotopic compositions along the Geochron are changing only slowly with time and it is unlikely that the Pb within the galena had isotopic compositions that exactly matched that of the Geochron (i.e. were derived from a primitive or undifferentiated source).

Modern mid-oceanic basalts plot to the radiogenic side of the geochron, consistent with a progressive increase in U/Pb in their depleted mantle source through time (see Sm-Nd). OIB (or plume) basalts have very radiogenic Pb, consistent with their derivation from recycled oceanic crust. U is thought to be preferentially incorporated into the oceanic crust during hydrothermal alteration on the ocean floor and during subduction.

There is a Pb paradox, in that a complimentary low U/Pb, U-donor reservoir, from which the U has been contributed to the ocean basalts, has not yet been identified (lower crust?, core?).

Whole-rock and mineral Pb-Pb isochrons

Both U and Pb have somewhat similar chemistries so these elements are not strongly fractionated from each other during magmatic or hydrothermal processes unless a high-U accessory mineral, such as zircon (Zr2SiO4) or baddeleyite (ZrO2), is crystallising. A sufficiently large range in the U/Pb ratio is not generated by basaltic magmatic processes to enable precise dating by the whole-rock Pb-Pb method. These elements are also generally mobile during hydrothermal metamorphism and alteration.

The rate of change of the abundance of the daughter isotope 207Pb is low because 235U has largely decayed away. Furthermore, only one Pb isotope (204Pb) is unaffected by radiogenic decay, so it is not possible to precisely correct Pb isotope ratios for instrumentally induced mass fractionation. These factors limit the precision and usefulness of whole-rock Pb-Pb isochrons.